Conventional cathode ray tubes (CRTs) are used in display monitors for computers, television sets, and other video devices to visually display information. Use of a luminescent phosphor coating on a transparent face, such as glass, allows the CRT to communicate qualities such as color, brightness, contrast and resolution which, together, form a picture for the benefit of a viewer.
Conventional CRTs have, among other things, the disadvantage of requiring significant physical depth, i.e. space behind the actual display screen, resulting in such units being large and cumbersome. There are a number of important applications in which this physical depth is deleterious. For example, the depth available for many compact portable computer displays precludes the use of conventional CRTs. Furthermore, portable computers cannot tolerate the additional weight and power consumption of conventional CRTs. To overcome these disadvantages, displays have been developed which do not have the depth, weight or power consumption of conventional CRTs. These "flat panel" displays have thus far been designed to use technologies such as passive or active matrix liquid-crystal displays ("LCD") or electroluminescent ("EL") or gas plasma displays.
A flat panel display fills the void left by conventional CRTs. However, the flat panel displays based on liquid-crystal technology either produce a picture that is degraded in its fidelity or is non-emissive. Some liquid-crystal displays have overcome the non-emissiveness problem by providing a backlight, but this has its own disadvantage of requiring more energy. Since portable computers typically operate on limited battery power, this becomes an extreme disadvantage. The performance of passive matrix LCDs may be improved by using active matrix LCD technology, but the manufacturing yield of such displays is very low due to required complex processing controls and tight tolerances. EL and gas plasma displays are brighter and more readable than liquid-crystal displays, but are more expensive and require a significant amount of energy to operate.
A solution has been found in field emission displays, which combine the visual display advantages of the conventional CRT with the depth, weight and power consumption advantages of more conventional flat panel liquid-crystal, EL and gas plasma displays. Within such field emission displays, electrons are emitted from a cold electron emitter electrode due to the presence of an electric field applied across the electrodes comprising the display, which bombard a phosphor coated anode, thereby generating light. Such a matrix-addressed flat panel display is taught in U.S. Pat. No. 5,015,912, which issued on May 14, 1991, to Spindt et al., which is hereby incorporated by reference herein, and which uses micro-tip cathodes of the field emission type.
Additionally, please refer to U.S. patent application Ser. No. 07/851,701, filed Mar. 16, 1992 and entitled "Flat Panel Display Based on Diamond Thin Films," U.S. patent application Ser. No. 07/993,863, filed Dec. 23, 1992 and entitled "Triode Structure Flat Panel Display Employing Flat Field Emission Cathodes," U.S. patent application Ser. No. 07/995,847, filed Dec. 23, 1992 and entitled "Diode Structure Flat Panel Display," U.S. patent application Ser. No. 08/071,157, filed Jun. 2, 1993, and entitled "Amorphic Diamond Film Flat Field Emission Cathode," U.S. Pat. No. 5,199,918, issued Apr. 6, 1993 and entitled "Method of Forming Film Emitter Device with Diamond Emission Tips,"and U.S. Pat. No. 5,312,514, issued May 17, 1994 and entitled "Method of Making a Field Emitter Device Using Randomly Located Nuclei as an Etch Mask," which applications and patents have been filed or have been granted to a common assignee and are hereby incorporated by reference herein, for further discussions on cold cathode field emission displays and related technology.
Field emission display panels have pixels that efficiently produce light at low level currents on the order of tens of microamps (".mu.A"). These pixels' voltage-to-current relationship may have random noise, threshold variations, soft forward knees and several hundred volt turn-on characteristics. Furthermore, the X-Y organization lines in the display have parasitic capacitances. Moreover, according to the well-known Fowler-Nordheim ("F-N") theory, the current density of field emissions changes by as much as 10 percent when cathode/anode separation changes by only 1 percent. Further, red, green and blue phosphors often have different efficiencies. All of these variations can cause adjacent or distant discreet pixels to have widely varying light outputs when driven with either constant currents or constant voltages. FIG. 5 illustrates an example of the fluctuations in the response of the current flowing across a pixel anode (corresponding to the emission of electrons from a cathode) as a function of the difference in the potentials between electrodes corresponding to a particular pixel within a matrix addressable flat panel display. Two curves are shown for two different screen pixels, A and B. The curves do not exhibit the same characteristics. It can be seen that for the same difference in potentials (Vop=80 volts), pixel A is brighter than pixel B.
In order to obtain a high yield for flat panel displays with large pixel counts, it is essential that variations in pixel performance be compensated so that variations in pixel uniformity can be tolerated. Otherwise, various pixels within the display that are less efficient, or less "hot," will not be as bright as other pixels when energized with the same amount of energy. Because of the essential purpose of a display, it is imperative that a display panel have no "bad" pixels.
Prior flat panel displays have not been completely successful in overcoming the problem of field emission variations. Other display technologies with highly non-linear electrical response characteristics have similar problems.
Thus, there is a need in the art for flat panel displays having substantially uniform pixel brightness. There is a further need in the art for a field emission flat panel display having pixels with substantially equal brightness capabilities and operating at constant voltages. There is also a need in the art for flat panel displays wherein variations and light output of pixels are equalized in the presence of parasitics that cause drive voltages to be slow and power consumptive.